The present invention relates to a magnetic resonance imaging apparatus (MRI apparatus) for obtaining tomograms of desired portions of an object to be examined using nuclear magnetic resonance (NMR). In particular, it relate to an MRI apparatus capable of obtaining desired range of images of excellent quality in minimal time to enable visualization of movement in the vascular system.
An MRI apparatus utilizes NMR to measure density distribution and relaxation time etc. of atomic nuclear spins (referred as spins hereinafter) in a desired portion of an object to be examined and displays images of desired slices of the object produced from the measured data. Conventional MRI apparatuses have a blood flow imaging function called MR angiography (MRA). This function includes a method using a contrast agent and a method using no contrast agent.
In a common method using a contrast agent, a gradient echo type sequence of short TR (repetition time) is used in combination with a T1-shortening type contrast agent such as Gd-DTPA. The method utilizes the fact that blood spins containing T1 shortening contrast agent are not likely to be saturated by the repeated excitation of a short TR because the blood spins have a shorter T1 than those of surrounding tissues and generate high-strength signals relative to the surrounding tissues, and enables to visualize blood vessels filled with blood containing a contrast agent with high contrast relative to the other tissues. Measurement of volume data including blood vessels, typically three-dimensional measurement, is conducted while the contrast agent remains in the blood of interest and the obtained three-dimensional images are combined and subjected to projection process to depict blood flow. In order to obtain information of wide range and high resolution, sequence based on the three-dimensional gradient echo method for obtaining three-dimensional data is generally employed.
In order to produce good images in such a three-dimensional contrast MRA, the following factors are important. (1) A manner of injecting a contrast agent, and (2) Measurement time or timing. With regard to (1), the contrast agent should be injected such that its high concentration is maintained in a blood vessel of interest in a stable manner. For this purpose, a rapid injection method using an automatic injection machine is generally utilized.
With regard to (2), if arteries are selectively imaged, for example, the imaging timing is determined such that the concentration of the contrast agent becomes high when data are acquired. Ideally, the concentration of the contrast agent reaches its peak value when a central part (low frequency region) of the k-space which controls the image contrast is sampled. The timing is determined corresponding to date-acquisition ordering of an employed pulse sequence.
Conventional data-acquisition ordering includes a sequential ordering in which data are acquired from one high-frequency side of k-space toward the other high-frequency side via a low-frequency region, and a centric ordering in which data are acquired from low-frequency region of the k-space toward both high-frequency sides alternately. Generally, the centric ordering is employed. In a centric ordering in a three-dimensional measurement, one of a phase encoding loop and a slice encoding loop is set to be an outer loop and the other to be an inner loop, and either or both is controlled by a centric ordering.
However, as shown in FIG. 1(b), the centric ordering in this case is not a centric ordering in a true sense because a distance between the k-space origin and a sampling point fluctuates, and therefore likely to be influenced by movement of an examined object and makes separation of arteries and veins insufficient.
For solving this problem, an elliptical centric ordering is proposed, in which sampling order is controlled in view of the relative FOV (Field of View) such that the distance from the ky-kz space origin to a sampling point increases as the measurement proceeds (FIG. 1(c)) (xe2x80x9cPerformance of an Elliptical Centric View Order for Signal Enhancement and Motion Artifact Suppression in Breath-hold Three-Dimensional Gradient Echo Imaging. Alan, et al. Magnetic Resonance in Medicine 38:793-802,1997xe2x80x9d)
This data-acquisition method enables to produce arterial images selectively by starting measurement at the time when the concentration of a contrast agent in the blood vessel of interest increases since low-frequency region data that dominates the image contrast are measured at the beginning of the measurement time.
Although the above-mentioned centric ordering and elliptical centric ordering enable to determine the image contrast in the early stage of measurement and are efficient for obtaining arterial images, if the optimal measuring moment is missed, the low-frequency information is acquired during the concentration of a contrast agent is low and thereby image quality becomes degraded. Especially, if the measuring time is too early, the low-frequency data is sampled in the time period when signals of a blood vessel are extremely low and the high-frequency region data is sampled in the time period when the signals of the blood vessel are high. This causes rinsing artifacts having no direct current component. In addition, the measurement time is prolonged because overall measurement is performed from the origin as a center toward the high-frequency region of the k-space horizontally or vertically.
On the other hand, the sequential ordering enables to produce stable images in which remarkable artifact is not likely to generate even if the measurement timing is somewhat wrong. However, this ordering is susceptible to movement of an examined object similarly to the aforementioned centric ordering and separation of artery and veins becomes insufficient.
Accordingly, an object of the present invention is to provide an MRI apparatus capable of visualizing a whole of a blood vessel of interest with high contrast in minimal time while reducing the influence of time shift (error) from an optimal measurement time. Another object of the present invention is to provide an MRI apparatus which is insusceptible to the influence of movement of an examined object and capable of visualizing arteries and veins separately. Yet another object of the present invention is to provide a data-acquisition method suitable for MRA.
In order to achieve the above-mentioned objects, an MRI apparatus of the present invention employs a data-acquisition method in which sampling points of k-space are divided into two groups and, in the first group which is measured first, sampling order is controlled from the high-frequency region toward the low-frequency region such that the distance from the k-space origin to a sampling point progressively decreases and, in the second group, sampling order is controlled in an opposite manner from the low-frequency region toward the high-frequency region such that the distance from the k-space origin to a sampling point progressively increases.
Specifically, an MRI apparatus of the present invention comprises static magnetic field generating means for generating a static magnetic field in a space where an object to be examined is placed, gradient magnetic field generating means for applying gradient magnetic fields in the slice direction, phase encoding direction and readout direction, transmitting means for applying high-frequency magnetic field to cause nuclear magnetic resonance in atomic nuclei of a living tissue of the object, receiving means for detecting echo signals emitted by the nuclear magnetic resonance, control means for controlling the magnetic field generating means, transmitting means and receiving means, signal processing means for performing image reconstruction operation using the echo signals detected by the receiving means, display means for displaying the produced image, wherein the control means performs a three-dimensional sequence including a slice encoding step and a phase encoding step, upon performing the sequence, divides sampling points of a k-space defined by a slice encode number and phase encode number into two groups and controls the gradient magnetic field generation means of slice direction and phase direction such that the distance from the origin of the k-space to a sampling point progressively decreases in the measurement of the first group and the distance from the origin of the k-space to a sampling point progressively increases in the measurement of the second group.
The manner of dividing the sampling points of the k-space into two group may be such that at least one of the groups includes sampling points from a low-frequency region to a high-frequency region and the other includes at least sampling points of a low-frequency region. A number of sampling points which are really measured may be the same or different in the two groups. Namely, either of the two groups may include non-sampling points (the points which are not measured).
Specifically, one embodiment is that sampling points are divided into two groups depending on the region of the k-space. According to this embodiment, the control system performs a three-dimensional sequence including a slice encoding step and a phase encoding step, upon performing the sequence, divides the k-space defined by the slice encode number and the phase encode number into two regions, and controls gradient magnetic field generating means of the slice direction and the phase direction such that, in one of the regions, the distance from the origin of the k-space to a sampling point progressively decreases and, in the other region, the distance from the origin of the k-space to a sampling point progressively increases.
In another embodiment, the sampling points of the k-space are divided into two groups which are in a relation of complex conjugate. According to this embodiment, the control system performs a three-dimensional sequence including a slice encoding step and a phase encoding step, upon performing the sequence, divides sampling points of the k-space defined by a slice encode number and phase encode number into two groups which share the origin and are in a relation of complex conjugate, and controls gradient magnetic field generating means of the slice direction and phase direction so that, in one of the groups, the distance from the origin of the k-space to a sampling point progressively decreases and, in the other group, the distance from the origin of the k-space to a sampling point progressively increases.
In this embodiment, it is preferable that adjacent two sampling points belong to different groups. In order to satisfy the condition that sampling points of the two groups are complex conjugate each other, some of the adjacent sampling points near the origin are required to belong the same group. Accordingly, in the specification, the wording xe2x80x9cto divide such that the adjacent sampling points belong to the different groupsxe2x80x9d means the state satisfying as much as possible the condition that the two groups are in a relation of complex conjugate and the adjacent sampling points belong to the different groups.
In yet another embodiment of the MRI apparatus of the present invention, the control system does not measure all of the sampling points in one of the two divided regions but measures a smaller number of sampling points than that of the other region. Or the control system does not measure all of the sampling points in one of the two groups but measures a smaller number of sampling points than that of the other group.
A three-dimensional image data-acquisition method of the present invention comprises, when a three-dimensional image data is acquired by repeating a plurality of times a step comprising selective excitation of a predetermined region of an object to be examined, application of encoding gradient magnetic fields at least in two directions, and collection of echo signals generated from the region while changing the intensities of the gradient magnetic fields,
divides a measurement space defined by the gradient magnetic fields encoding in the two directions, and
performs measurement of the tow divided regions sequentially such that the distance from the origin of the measurement space to a sampling point gradually decreases in the measurement of a region which is measured first and the distance from the origin of the measurement space to a sampling point gradually increases in the measurement of the region which is measured thereafter.
In the data-acquisition method of the present invention, when there are several sampling points whose distances from the origin are the same, the nearest sampling point from the latest measured point in the k-space is preferably measured next.
A three-dimensional image data-acquisition method of the present invention, when three-dimensional image data is acquired by repeating a plurality of times a step comprising selective excitation of a predetermined region of an object to be examined, application of encoding gradient magnetic fields at least in two directions, and collection of echo signals emitted from the region while changing the intensity of the gradient magnetic fields,
divides sampling points on a measurement space defined by the intensities of the encoding gradient magnetic fields of the two directions into two groups such that the first group and the second group share the origin and that adjacent and conjugating sampling points belong to different groups, and
performs measurement of the first group and the second group in this order such that, in a measurement of the first group, the distance of a sampling point from the origin of the measurement space progressively decreases and, in a measurement of the second group, the distance of a sampling point from the origin of the measurement space progressively increases.
According to the data-acquisition method of the present invention, as shown in FIG. 1(a), measurement can be performed without fluctuation of distance from the origin and a blood vessel concerned can be visualized with high contrast by making the time of measuring the lowest frequency component coincide with the time when the contrast agent enhance the signal intensity of the blood vessel most. In addition, even the time of measuring the low-frequency component is somewhat shifted from the peak of the signal intensity, proper sampling of the low-frequency component can be assured and images are not degraded.
For reference, variations of distance from the origin of the k-space in the conventional centric ordering, elliptical centric ordering and sequential ordering are shown in FIG. 1(b)-(d).
According to a preferable embodiment of the data-acquisition method of the present invention, a part of all of the sampling points is measured in the first group and all of the sampling points are measured in the second group.
Since the two groups are in a relation of complex conjugate, even if a part of one group is not measured, data which are not measured can be estimated from data of the other group. Particularly when a measurement of the first group is performed during the density of the contrast agent is increasing to its peak, sampling of unnecessary data having low signal intensity is avoided and thereby images of good quality can be obtained.